1. Field of the Invention
The present invention relates to an image forming apparatus connected to a sheet post processing device that carries out post processing for a sheet with an image formed thereon, and is configured to discharge the sheet at one of a plurality of discharging speeds.
2. Description of the Related Art
When a digital multifunction apparatus is used as a printer, data is typically printed in order beginning with the first page. For example, to print five pages, the pages are printed in order of the first page, second page, third page, fourth page, and fifth page. To stack the output sheets on a discharge tray in the correct order, each sheet has to be discharged with an image-formed surface facing the lower side. Accordingly, the image forming apparatus has a sheet reversing mechanism that reverses the front and back surfaces of a sheet.
The sheet reversing mechanism uses a switchback system in which a sheet is led to a reverse conveying path diverging from a straight discharge conveying path, then the sheet conveying direction of the sheet is changed, and the sheet is conveyed. In this case, to prevent continuously conveyed sheets from colliding with one another in the reverse conveying path, at least a sheet interval for a switchback distance is required. In order to increase productivity by decreasing the sheet interval, Japanese Patent Laid-Open No. 6-161185 describes control that increases a speed of a sheet from start of reversing a sheet until discharge of the sheet from an image forming apparatus body. A discharging speed when a sheet is discharged in reverse manner is different from a discharging speed when a sheet is discharged in non-reverse manner.
Further, when the discharging speed is changed, a conveying speed of a sheet post processing device that receives a sheet discharged from the image forming apparatus has to be changed in accordance with the discharging speed of the image forming apparatus body.
For example, if the discharging speed of the apparatus body is higher than the receiving speed of the sheet post processing device, a sheet may be pushed in an area between the sheet post processing device and the image forming apparatus, resulting in the sheet not be conveyed in an ordinary way.
In contrast, if the receiving speed of the sheet post processing device is higher than the discharging speed of the apparatus body, the following restrictions may be conceived. If the conveying speed of a sheet is changed while a toner image is transferred on the sheet or fixed to the sheet, changing of the speed may adversely affect image formation. Thus, the speed should not be changed during this period. In particular, if a distance relationship is established such that the leading edge of a sheet reaches the sheet post processing device although the trailing edge of the sheet has not passed through a fixing unit, the receiving speed of the sheet post processing device should not be higher than the discharging speed of the apparatus body.
Also, U.S. Pat. No. 6,139,012 describes a configuration that includes a conveying path having a distance relationship such that the leading edge of a sheet reaches a sheet post processing device after the trailing edge of the sheet has passed through a fixing unit. In this case, even if the conveying speed of the sheet post processing device is different from that of the image forming apparatus, a sheet is pulled out while the sheet slips on rollers of the image forming apparatus. Thus, the sheet can be delivered.
However, in the '012 Patent, immediately after an image is fixed to the sheet, the sheet is conveyed in a slipping manner. Implementation of this arrangement makes assuring quality of an image difficult.
Therefore, it is necessary to control the conveying speed of the sheet post processing device and the discharging speed of the image forming apparatus body so as to be at substantially equivalent speeds.
In the field of commercial printing, a set of printed sheets may include a sheet with an image formed on only one surface and a sheet with images formed on both surfaces.
FIG. 11 illustrates a discharging operation of related art in a case in which a sheet with an image formed on only one surface and a sheet with images formed on both surfaces are mixed. For example, in an image forming apparatus that discharges a sheet in reverse manner at 1000 mm/s and discharges a sheet in non-reverse manner during duplex printing at 500 mm/s, image formation is carried out in order of one-side printing (reverse discharge), duplex printing (non-reverse discharge), one-side printing (reverse discharge), and then duplex printing (non-reverse discharge). In this case, the conveying speed of the sheet post processing device has to be changed when discharge is changed from reverse discharge to non-reverse discharge, and when discharge is changed from non-reverse discharge to reverse discharge.
However, to change the speed in a short time, a costly motor having a large torque is required. Such a motor increasing the cost of the sheet post processing device.
Also, if a plurality of sheet post processing devices are connected, a sheet interval has to be large to allow a sheet post processing device, which requires the longest time for changing the speed, to change the speed. This significantly decreases productivity.
Thus, if one-side printing and duplex printing are alternately carried out as shown in FIG. 11, the sheet interval will be large every time a sheet is discharged due to the time needed to change the speed required by the sheet post processing device.
If all sheets including sheets for one-side printing are printed by duplex printing, the speed does not have to be changed in the sheet post processing device. However, if the proportion of sheets for one-side printing is large, productivity is decreased because the sheets are conveyed through a path for duplex printing.